For a housing or decoration of an electronic device such as a portable phone, an appropriate material is selected and used from materials such as resin and metal in consideration of various factors such as decorativeness, scratch resistance, workability, and cost.
In recent years, there have been attempts to use, as a material for housing, a glass that has not been used hitherto. According to Patent Reference 1 (Japanese Patent Application Laid-open No. 2009-061730), by forming the housing itself from a glass in an electronic device such as a portable phone, it is possible to exhibit a unique decorative effect with transparency.
The housing or decoration of an electronic device for portable use such as a portable phone is required to have high strength in consideration of breakage by an impact of dropping when in use or contact scratches due to long-term use.
As a method to increase strength of the glass, a method of forming a compressive stress layer on a glass surface is generally known. Representative methods to form the compressive stress layer on a glass surface are an air-cooling tempering method (physical tempering method) and a chemical strengthening method. The air-cooling tempering method (physical tempering method) is performed by rapidly cooling such as air cooling or the like a glass plate surface heated to a temperature near a softening point. On the other hand, the chemical strengthening method is to replace alkali metal ions (typically, Li ions, Na ions) having a smaller ion radius existing on the glass plate surface with alkali ions (typically, Na ions or K ions for Li ions, or K ions for Na ions) having a larger ion radius by ion exchange at temperatures lower than or equal to a glass transition point.
For example, in general, the glass for decoration as described above is often used with a thickness of 2 mm or less. When the air-cooling tempering method is employed for such a thin glass plate, it is difficult to assure a temperature difference between the surface and the inside, and hence it is difficult to form the compressive stress layer. Thus, in the glass after being tempered, the intended high strength characteristic cannot be obtained.
Further, in the air-cooling tempering, due to variation in cooling temperature, there is a great concern that the flatness of the glass plate is impaired. The concern that the flatness is impaired is large in a thin glass plate in particular, and there is a possibility of impairing texture aimed by the present invention. From these points, it is preferred that the glass plate be tempered by the latter chemical strengthening method.
Further, in the housing or decoration of an electronic device such as a portable phone, a black glass is widely used which does not strongly emphasize the presence of the device itself, and by which firmness and luxuriousness can be obtained simultaneously.
A glass described in Patent Reference 2 (Japanese Examined Patent Application Publication No. S45-016112) has been known as a glass that can be chemically strengthened and exhibits a black color. The glass described in Patent Reference 2 is an aluminosilicate glass containing a high concentration of iron oxide.
In the example disclosed in above Patent Reference 2, arsenous acid is used as a refining agent. The arsenous acid is an environment-affecting substance whose inverse effects to the environment are concerned not only in manufacturing processes but through the lifecycle of the product.
Accordingly, the inventors of the present invention heated and melted a glass material of the composition disclosed in the example of Patent Reference 2 without adding the arsenous acid, and found that only a glass can be obtained which hardly release bubbles, that is, has a poor refining ability, and hence has many remaining bubbles. Specifically, after a molten glass was casted in a block shape and was sliced into a plate shape and the surface thereof was polished, it was recognized that a large number of pockmark-like dents (hereinafter referred to as open bubbles) formed by bubbles being cut in the glass is exposed on the polished surface.
For the purposes of housing or decoration of an electronic device as described above, a glass in which open bubbles exist cannot be used due to the demand for improving appearance quality, and thus causes a problem of largely reducing the production yield. There is also a concern that the open bubbles become an origin of crack and decrease the strength.
Moreover, the housing of an electronic device may be shaped and used not only in a flat plate shape but also in a concave or convex shape. Thus, a glass which is easily press-formed is demanded.
For the purpose of confirming that it has strength of a certain degree or more in quality management, a compressive stress amount of the chemical strengthened glass is also measured.
However, when the glass has a black color, if it is measured with an existing surface stress meter, there is a problem that the measurement light is absorbed by the glass and the measurement of compressive stress amount cannot be performed. Accordingly, it is demanded that even such a glass having a black color tone passes a certain amount or more of light having a wavelength out of the visible range.
It is an object of the present invention to provide a glass for chemical strengthening excelling in characteristics preferred for the purposes of housing or decoration of an electronic device, that is, bubble quality, strength, and light transmission characteristics.